Device and method for frequency conversion of electromagnetic radiation

Abstract

The invention relates to a device for frequency conversion of electromagnetic radiation, which comprises a non-linear medium. The device is designed to superimpose in the non-linear medium an electromagnetic input radiation having a first wavelength with an electromagnetic driver radiation having a second wavelength along a propagation path in order to generate an electromagnetic output radiation having a third wavelength. The second wavelength is longer than the first wavelength. Furthermore, the non-linear medium is designed for a free propagation of the driver radiation in the non-linear medium.

Description

[0001] Exemplary embodiments of the present invention relate to a device for frequency conversion of electromagnetic radiation, in particular a frequency converter for the conversion of single photons, especially a quantum frequency converter. Further exemplary embodiments relate to a method for frequency conversion of electromagnetic radiation.

[0002] Current research focuses on the implementation of networks between quantum computers. Since the fluorescence wavelengths of the quantum bits (qubits, the processing units of a quantum computer) are not typically in the range of telecommunications wavelengths (1260 nm - 1675 nm), a conversion step is necessary to reduce propagation losses during transmission through optical fibers [1].

[0003] Frequency conversion of these optical signals places specific demands on the frequency converter that are not relevant in classical frequency conversion. Unlike classical optical communication, the information to be transmitted is encoded in individual photons and not in fields of macroscopic intensity. Therefore, the fidelity of the transmission is reduced by each additional photon of the same wavelength introduced (i.e., from another source). These photons are called noise photons.

[0004] The rate of introduced noise photons should therefore be low at every point along the transmission path. This applies particularly to the conversion process, which can be implemented, for example, by superimposing the photons to be converted with another field in a nonlinear medium.

[0005] In existing frequency converters, the conversion process takes place in waveguide structures in periodically polarized nonlinear media, e.g., in lithium niobate [2], [3], [4], [5] or in potassium titanyl phosphate [6]. In addition to the radiation field to be converted (the "input signal" or "field" with wavelength λin), another field of high average power is incident, resulting in optical-optical conversion efficiencies on the order of quantum efficiency (nQ = λout / λin). Here, λout denotes the wavelength of the converted field ("output signal" or "field").

[0006] However, the presence of a powerful field in a nonlinear medium causes photons to be generated in all spectral ranges of longer wavelengths, and therefore also in the output channel, particularly due to a process known as "spontaneous parametric down-conversion" (SPDC).

[0007] The rate of generated noise photons is therefore at least 150 Hz / pm in existing systems, e.g., based on the half-width of the spectral filters of the optical output.

[0008] Considering the current state of the art, a frequency conversion concept that offers an improved ratio between high conversion efficiency and a low noise photon rate would be desirable. In particular, it would be desirable for the concept to exhibit low efficiency of the SPDC process in generating photons of the output wavelength, yet still offer a conversion efficiency on the order of quantum efficiency.

[0009] DE 10 2019 131827 A1 discloses all features of the preamble of claim 1.

[0010] The present invention is based on the understanding that efficient and low-noise frequency conversion can be achieved by superimposing electromagnetic input radiation with electromagnetic driver radiation along a propagation path in a nonlinear medium. This is accomplished by eliminating the need for waveguides of the driver radiation in the nonlinear medium and by selecting a wavelength of the driver radiation larger than that of the input radiation. Inaccuracies in the fabrication of a waveguide represent a significant source of noise photons in conventional frequency converters. However, it has previously been assumed that a waveguide-free implementation is disadvantageous with regard to the higher power densities required for the driver radiation, since the interaction of a high-power driver radiation with the nonlinear medium can generate a high rate of noise photons.In contrast, the inventors recognized that by using a longer wavelength of the driver radiation compared to the wavelength of the input radiation, phonon bands generated by the driver radiation are less pronounced or do not fall within the wavelength range of the output radiation, thus enabling low-noise conversion. Although in a conversion process based on three-wave mixing, choosing the shortest wavelength of the driver radiation would be expected to result in higher conversion efficiency, it was recognized that, given the conflicting goals of high conversion efficiency and a low noise photon rate, it is advantageous to operate the conversion based on a driver radiation with a wavelength larger than that of the input radiation. This even allows for a power density of the driver radiation with which high conversion efficiency can be achieved without waveguiding.Examples of implementations thus enable low-noise and efficient photon conversion, resulting in a good signal-to-noise ratio.

[0011] Exemplary embodiments of the present invention provide a device for frequency conversion of electromagnetic radiation, for example, a device for frequency conversion of single photons, in particular for quantum information-preserving frequency conversion of single photons. The device comprises a nonlinear medium. The device is configured to superimpose electromagnetic input radiation with a first wavelength in the nonlinear medium with electromagnetic driver radiation of a second wavelength along a propagation path in order to generate electromagnetic output radiation with a third wavelength. For example, the device is configured to superimpose or mix the driver radiation and the input radiation collinearly along the propagation path. According to the invention, the second wavelength is greater than the first wavelength.Furthermore, the nonlinear medium is designed to allow free propagation of the driver radiation within or through the nonlinear medium. This means, for example, that the nonlinear medium (or the device) does not possess any waveguiding properties or a waveguide structure for the propagation of the driver radiation along the propagation path, such as properties that spatially confine the driver radiation as it propagates along the path. Therefore, the device can be designed to allow the driver radiation to propagate freely within the nonlinear medium without waveguiding.

[0012] According to exemplary embodiments, the device further comprises a resonator resonant for the second wavelength. The propagation path of the nonlinear medium, that is, the propagation path in the nonlinear medium along which the driver radiation and the input radiation are superimposed, is arranged in the resonator to provide the driver radiation along the propagation path. For example, the propagation path is arranged in an optical path of the resonator. The use of a resonator represents an energy-efficient way to provide a high power density of the driver radiation in the nonlinear medium. A high intensity of the driver radiation ensures a high conversion efficiency of a second-order nonlinear conversion process, in particular a three-wave mixing of photons from the input radiation, the driver radiation, and the output radiation.The use of a resonator makes it possible to provide the driver radiation with high power, high beam quality, and a narrow spectral bandwidth. In particular, this can be achieved cost-effectively using the resonator.

[0013] According to exemplary embodiments, the second wavelength is smaller than the third wavelength. Therefore, according to these embodiments, the second wavelength can be the mean wavelength in a three-wave mixture. For these cases of the ratio of the three wavelengths, the inventive implementation of frequency conversion with free propagation in the nonlinear medium is particularly advantageous, since a low noise photon rate can be achieved even though the wavelength of the driver radiation is shorter than the wavelength of the output radiation.

[0014] According to exemplary embodiments, the device is configured to focus the driver radiation in the nonlinear medium onto a focus with a cross-sectional area of ​​2*w₀. For example, the cross-sectional area is a maximum size of a beam cross-section at the focus; e.g., w₀ is the radius of the focus in the case of a circular beam cross-section. According to these exemplary embodiments, a length L of the nonlinear medium along the propagation path satisfies the condition L ≤ z R ∗ 1 3 D w 0 2 − 4 , where zR denotes the Rayleigh length for the driver radiation, and where D denotes a dimension of an aperture of the nonlinear medium, for example, a dimension in the spatial direction in which w0 is measured. The aperture can, for example, denote a cross-section or a diameter of a free surface of an entrance facet through which the propagation path passes, or through which the driver radiation and the input radiation enter the nonlinear medium. For example, D can denote the smallest free diameter of the aperture. Such a length L of the nonlinear medium ensures that the driver radiation is not reflected at interfaces of the nonlinear medium parallel to the propagation path, but instead exits the nonlinear medium.

[0015] According to exemplary embodiments, the nonlinear medium has a length of 2 mm to 60 mm along the propagation path. A short length allows for very precise phase matching between the input radiation, the driver radiation, and the output radiation along the length of the propagation path. Conversely, a long length makes it possible to position the focus of the driver radiation, the input radiation, and the output radiation further away from an inlet facet and an outlet facet of the nonlinear medium, thus keeping the power density at the inlet and outlet facets low. Furthermore, the conversion efficiency also increases with increasing interaction length up to a point where the phase relationship of the three fields can have a detrimental effect.

[0016] According to exemplary embodiments, the device is designed to focus the driver radiation in the nonlinear medium. The Rayleigh length of the focus of the driver radiation in the nonlinear medium is at most the Rayleigh length of a corresponding Gaussian beam, that is, a Gaussian beam of the same wavelength and cross-sectional area at the focus as the driver radiation. In other words, no waveguiding takes place in the nonlinear medium.

[0017] According to exemplary embodiments, the device is configured to focus the driver radiation onto a focus in the nonlinear medium. The device is designed to provide the driver radiation at the focus with a power density, for example, a power per unit area of ​​the beam cross-section at the focus, i.e., for example, an average power density averaged over the beam cross-sectional area, of at least 2 / π · 10⁵ < W / cm² < or at least 2 / π · 10⁶ < W / cm² <. At a power density above this limit, a particularly high efficiency of the conversion process can be achieved, especially on the order of quantum efficiency. Up to now, it has been assumed that the high power density required for frequency conversion without a waveguide structure precludes its implementation without a waveguide structure.Firstly, the required power density was considered so high that it was anticipated to destroy the nonlinear medium. Secondly, the temperature increase in the nonlinear medium caused by the high power density was considered detrimental to the conversion process. However, it was recognized that a high power density could achieve high conversion efficiency even without waveguides, and that this, particularly in combination with a longer wavelength driver radiation compared to the input radiation, could result in an improved compromise between conversion efficiency and noise.

[0018] According to exemplary embodiments, the aperture of the nonlinear medium in a first direction has a size of 0.5 mm to 10 mm. Furthermore, the aperture in a second direction, perpendicular to the first direction, has a size of 0.5 mm to 10 mm. An aperture with a size of at least 0.5 mm ensures a low power density at an inlet facet and / or an outlet facet of the nonlinear medium. An aperture of no more than 10 mm ensures a cost-effective and space-saving implementation of the device.

[0019] According to exemplary embodiments, the nonlinear medium exhibits a uniform second-order electrical susceptibility along the propagation path. For periodically polarized nonlinear media, inaccuracies in the periodicity of the nonlinear medium's polarity can contribute to the generation of noise photons. Therefore, a particularly low-noise conversion can be achieved if the nonlinear medium exhibits a uniform polarity along the propagation path.

[0020] According to exemplary embodiments, the second-order electrical susceptibility of the nonlinear medium exhibits a periodically changing sign along the propagation path. This periodically changing polarity of the nonlinear medium along the propagation path allows for particularly good phase matching of the incoming, outgoing, and driving radiation, especially for long lengths of the nonlinear medium. Precise phase matching increases the efficiency of the conversion process, potentially allowing, for example, a lower power density to be sufficient.

[0021] In some embodiments, the propagation path runs between an inlet facet and an outlet facet of the nonlinear medium. For example, the propagation path runs from the inlet facet to the outlet facet of the nonlinear medium. In these embodiments, the inlet facet forms an angle with a plane perpendicular to the propagation path in the range of 0° to 10°. Furthermore, the outlet facet forms an angle with a plane perpendicular to the propagation path in the range of 0° to 10°. In some examples, the angle for the inlet facet and / or for the outlet facet is greater than 0°. For example, the angles of the inlet facet and the outlet facet are in the range of 2° to 10°.An angle greater than 0° prevents backward propagation of the driver radiation, meaning that electromagnetic radiation propagated along the propagation path through the nonlinear medium and reflected at the exit facet of the nonlinear medium cannot travel in the opposite direction of the propagation path.

[0022] According to exemplary embodiments, the device is designed to superimpose the input radiation in the nonlinear medium with the driver radiation in such a way that the input radiation and the driver radiation coincide in their beam directions. This coincidence of beam directions ensures high efficiency of the conversion process.

[0023] According to the exemplary embodiments, the third wavelength lies in the range of 1260 nm to 1675 nm. The sum of the reciprocals of the second and third wavelengths corresponds to the reciprocal of the first wavelength. Electromagnetic radiation with a wavelength in this range can be transmitted with particularly low loss using waveguides, making these wavelength ranges well-suited for long-distance communication.

[0024] According to exemplary embodiments, the device comprises a further resonator resonant for the second wavelength. A further propagation path is arranged within this further resonator. This further propagation path extends through a further nonlinear medium. According to these exemplary embodiments, the device is configured to superimpose a further electromagnetic input radiation with a further electromagnetic driver radiation of the second wavelength along the further propagation path in order to generate a further electromagnetic output radiation. According to these exemplary embodiments, the device is configured to use a portion of the driver radiation coupled out from the resonator to provide the further driver radiation in the further resonator.In other words, the device can have multiple frequency converter modules, each configured to convert a specific input radiation into a specific output radiation. Using the radiation coupled out of the resonator in the subsequent resonator enables energy-efficient operation of multiple frequency converters. In particular, energy-efficient operation is achieved because the wavelength of the driver radiation is longer than the wavelength of the input radiation. Therefore, the intensity of the driver radiation is not reduced, but rather increased, during the conversion of the input radiation into the output radiation.

[0025] According to exemplary embodiments, the device further comprises an amplifier configured to amplify the portion of the driver radiation coupled out of the resonator and to supply the amplified driver radiation to the second resonator, so that the driver radiation in the first resonator and the driver radiation in the second resonator have the same intensity. Identical intensities in the resonators can ensure identical efficiency of the conversion process of the respective nonlinear media.

[0026] According to exemplary embodiments, the material of the nonlinear medium is a mixture of beta-barium borate, beta-BaB₂O₄, lithium niobate, LiNbO₃ or MgO:LiNbO₃, lithium tantalate, LiTaO₃ or MgO:LiTaO₃, potassium titanyl arsenate, KTiOAsO₄, potassium titanyl phosphate, KTiOPO₄, lithium triborate, LiB₃O₅, and bismuth triborate, BiB₃O₆. These materials offer high efficiency for second-order conversion processes.

[0027] According to exemplary embodiments, the third wavelength lies in the range of 1260 nm to 1675 nm, and the first wavelength in the range of 200 nm to half the third wavelength. Emission wavelengths of quantum bits can be particularly well realized in this range for the first wavelength. In particular, the first wavelength can lie in the range of 600 nm to 650 nm. In the latter range, quantum bits can be particularly advantageously implemented in the form of nitrogen vacancies. Especially in combination with the aforementioned materials, conversion in these wavelength ranges can be advantageously implemented.

[0028] According to exemplary embodiments, the device is configured to maintain the temperature of the nonlinear medium at a temperature T during operation, where T is between 0 °C and 200 °C. Furthermore, the material of the nonlinear medium according to these exemplary embodiments is KTA. A first crystallographic axis of the nonlinear medium forms an angle θ = 90° with the propagation path. Furthermore, a projection of the propagation path into a plane spanned by a second crystallographic axis and a third crystallographic axis of the nonlinear medium forms an angle Φ with the second crystallographic axis. The refractive index along the first crystallographic axis is greater than the refractive index along the second crystallographic axis, and vice versa.Furthermore, the first wavelength lies in the range of 350 nm to 840 nm and the third wavelength in the range of 1260 nm to 1675 nm. The sum of the reciprocals of the second and third wavelengths is equal to the reciprocal of the first wavelength. Furthermore, the second wavelength is smaller than the third wavelength. The following also applies to the angle Φ: . ϕ = ϕ 0 + ∂ ϕ ∂ λ Δλ + ∂ ϕ ∂ λ ∂ λ ∂ T ΔT , with T = 80°C + ΔT; λ = 1064nm + Δλ; ϕ 0 = 33° ± 1°; ∂ ϕ ∂ λ = − 0 , 9 ° / nm ± 0 , 3 ° / nm ; and ∂ λ ∂ T = 0 , 05 nm / ° C ± 0 , 025 nm / ° C .

[0029] The inventors recognized that KTA is particularly well-suited for frequency conversion, as highly efficient conversion is possible along the directions defined above. This enables low-noise and efficient conversion, especially for the specified wavelength ranges for converting stable qubit wavelengths to the telecom wavelength range. Due to the high conversion efficiency, the power density of the driver radiation can be kept low enough to ensure stability of the nonlinear medium and, furthermore, low-noise conversion. It was thus realized that a conversion efficiency on the order of quantum efficiency can be achieved without using a waveguide structure if the propagation path in KTA is chosen along a specific orientation.

[0030] According to exemplary embodiments, the device is configured to maintain the temperature of the nonlinear medium at a temperature T during operation, where T is between 0 °C and 200 °C. Furthermore, the material of the nonlinear medium according to these exemplary embodiments is KTA. A first crystallographic axis of the nonlinear medium forms an angle θ = 90° with the propagation path. Furthermore, a projection of the propagation path into a plane spanned by a second crystallographic axis and a third crystallographic axis of the nonlinear medium forms an angle Φ with the second crystallographic axis. The refractive index along the first crystallographic axis is greater than the refractive index along the second crystallographic axis, and vice versa.Furthermore, the first wavelength lies in the range of 600 nm to 700 nm and the third wavelength in the range of 1260 nm to 1675 nm. Alternatively, the first wavelength lies in the range of 635 nm to 640 nm and the third wavelength in the range of 1260 nm to 1675 nm. Alternatively, the first wavelength lies in the range of 635 nm to 640 nm and the third wavelength in the range of 1530 nm to 1675 nm. In all these alternatives, the sum of the reciprocals of the second and third wavelengths equals the reciprocal of the first wavelength. Furthermore, the second wavelength is smaller than the third wavelength. Furthermore, the following applies to the angle Φ: . ϕ = ϕ 0 + ∂ ϕ ∂ λ Δλ + ∂ ϕ ∂ λ ∂ λ ∂ T ΔT , with T = 80°C + ΔT; λ = 1064nm + Δλ; ϕ 0 = 33° ± 1°; ∂ ϕ ∂ λ = − 0 , 9 ° / nm ± 0 , 1 ° / nm ; and ∂ λ ∂ T = 0 , 05 nm / ° C ± 0 , 015 nm / ° C The first of the alternatives is particularly suitable for converting the wavelengths of defect qbits into the telecom frequency band. The second of the alternatives is particularly suitable for converting the wavelengths of nitrogen defect qbits into the telecom frequency band. The third of the alternatives is particularly suitable for converting the wavelengths of nitrogen defect qbits into the C-, L-, or U-band using an Nd laser.

[0031] According to exemplary embodiments, the device is designed to convert individual photons of the input radiation into photons of the output radiation while preserving quantum information. For example, the rate of noise photons, i.e., photons of the third wavelength that do not result from the conversion of a photon of the input radiation in a three-wave mixture, normalized to the conversion efficiency from input radiation to output radiation and to the bandwidth of the output radiation, is at most 6.12 Hz / pm / %.

[0032] Further embodiments of the present invention provide a method for frequency conversion of electromagnetic radiation. The method involves superimposing electromagnetic input radiation with a first wavelength with electromagnetic driver radiation of a second wavelength along a propagation path in a nonlinear medium to generate electromagnetic output radiation with a third wavelength. The second wavelength is longer than the first. Furthermore, the superimposition in the nonlinear medium is carried out such that the driver radiation propagates freely within the nonlinear medium.

[0033] Exemplary embodiments of the invention are described below with reference to the accompanying figures. These show: Fig. 1 illustrates a device for frequency conversion of electromagnetic radiation according to an embodiment, Fig. 2 illustrates a beam path in the nonlinear medium according to embodiments, Fig. 3 illustrates an example of the nonlinear medium with angled facets, Fig. 4 illustrates an example of an orientation of the propagation path with respect to crystallographic axes of the nonlinear medium, Fig. 5 illustrates an example of the nonlinear medium with periodically changing polarity, Fig. 6 illustrates an example of the device with a resonator, Fig. 7 illustrates examples of the resonator, Fig. 8 illustrates examples of beam paths in the resonator, Fig. 9 illustrates an example of a device for frequency conversion of electromagnetic radiation with reuse of the driver radiation, Fig. 10 shows a flowchart of a method for frequency conversion according to an embodiment.

[0034] Exemplary embodiments of the present invention are described in detail below, using the accompanying descriptions. Many details are described in the following description to provide a more thorough explanation of exemplary embodiments of the invention. However, it is obvious to those skilled in the art that other exemplary embodiments can be implemented without these specific details. Features of the different described exemplary embodiments can be combined with one another, unless features of such a combination are mutually exclusive or such a combination is expressly excluded.

[0035] It should be noted that identical or similar elements, or elements with the same functionality, may be given the same or similar reference symbols or be labelled identically, whereby a repeated description of elements with the same or similar reference symbols or labelled identically is typically omitted. Descriptions of elements with the same or similar reference symbols or labelled identically are interchangeable.

[0036] Fig. 1 Figure 10 illustrates a device 10 for frequency conversion of electromagnetic radiation according to an exemplary embodiment. For example, the device 10 can function as a quantum frequency converter. The device 10 comprises a nonlinear medium 40. The device 10 is configured to superimpose electromagnetic input radiation 21 in the nonlinear medium 40 with electromagnetic driver radiation 23 along a propagation path 30. For example, the device 10 can provide the driver radiation 23 on the propagation path 30 in the nonlinear medium 40. Furthermore, the device 10 can be configured to provide the input radiation 21 on the propagation path 30 in the nonlinear medium 40. The input radiation 21 has a first wavelength λ1. The driver radiation has a second wavelength λ2, which is larger than the first wavelength.The output radiation 22 can be generated in the nonlinear medium by superimposing the input radiation 21 and the driver radiation 23. The output radiation 22 has a third wavelength λ3. The nonlinear medium 40 is designed to allow free propagation of the driver radiation 23, and optionally also of the input radiation 21 and the output radiation 22.

[0037] The intensity of the input radiation 21 can be lower than the intensity of the driver radiation 23. For example, the input radiation 21 consists of photons generated using quantum emitters. It can, for instance, be single photons, meaning the input radiation can have a rate of less than 100,000 Hz. Therefore, the power of the input radiation 21 can be lower than the power of the driver radiation 23 by at least a factor of 10⁸ or 10¹⁰.

[0038] For example, the generation of output radiation 22 based on input radiation 21 can occur via a nonlinear conversion process, e.g., second order, in particular a three-wave mixing process. Accordingly, a relationship between the wavelengths λ₁, λ₂, and λ₃ can arise from energy conservation. By superimposing input radiation 21 with driver radiation 30, a photon of input radiation 21 can be converted into a photon of output radiation 22 with a high probability, i.e., with a high conversion efficiency. The participation of a photon of driver radiation 30 can ensure energy conservation in the conversion process. The probability of this three-wave mixing increases with increasing density of photons of driver radiation 30.Accordingly, the device 10 can be configured to focus the input radiation 21 and the driver radiation 23 at a common point onto the propagation path 30 in the nonlinear medium 40. For this purpose, the device 10 can, for example, include focusing optical elements, such as lenses, which can be arranged, for example, in a beam path of the driver radiation 23 and the input radiation 21 before they enter the nonlinear medium 40. Similarly, the device 10 can include optical elements in the beam path after the nonlinear medium for collimating the driver radiation 23 and the output radiation 22.

[0039] According to exemplary embodiments, the first wavelength can lie in a range from 190 nm to 1260 nm. Furthermore, the third wavelength can lie in a range from 1260 nm to 1675 nm. In these embodiments, as well as in others, the sum of the reciprocals of the second and third wavelengths can correspond to the reciprocal of the first wavelength. The wavelength of the driver radiation 23 can therefore be adapted to the first wavelength λ1 and the third wavelength λ3 such that the condition of energy conservation in the conversion process is met.

[0040] According to some embodiments, the second wavelength is smaller than the third wavelength. According to these embodiments, the second wavelength is therefore the average wavelength in the three-wave mixing process of the input radiation, the output radiation, and the driver radiation.

[0041] Examples of materials of low-linear mediums 40 are Beta-Bariumborate (BBO), beta-BaB 2 O 4 , Lithium-Niobate (LN), LiNbO 3 or MgO:LiNbO 3 , Lithium Tantalate LiTaO 3 or MgO: LiTaO 3 , Calcium (KTA), KTiOAsO 4 , Potassium titanyl phosphate (KTP), KTiOPO 4 , Lithiumtriborate (LBO), LiB 3 O 5 , und Bismuth-Triborate (BiBO), BiB 3 O 6 .

[0042] In exemplary embodiments where the nonlinear medium material is one of those mentioned, the third wavelength lies in the range of 1260 nm to 1675 nm. Furthermore, the first wavelength lies in the range of 200 nm to half the third wavelength. That is, the first wavelength is greater than 350 nm and the second wavelength is less than the third wavelength. In exemplary embodiments, the first wavelength lies in the range of 600 nm to 650 nm. In particular, the first wavelength is advantageously located above the short-wavelength band edge of the nonlinear medium material. This is at 350 nm for KTA and KTP, at 185 nm for beta-BaB₂O₄, at 280 nm for LiTaO₃, at 330 nm for LiNbO₃, at 286 nm for BiBO, and at 160 nm for LBO.

[0043] According to exemplary embodiments, the nonlinear medium exhibits uniform polarity along the propagation path, i.e., for example, a uniform second-order electrical susceptibility. The nonlinear medium can, for example, have a continuous crystal structure.

[0044] Optionally, the device can further include a filter arrangement 78, which is arranged in a beam path of the output radiation 22. The filter arrangement is designed to transmit the output radiation 22 and to suppress the driver radiation 23. This allows the output radiation to be separated from the driver radiation in order to further use the output radiation, for example, for transmission.

[0045] According to exemplary embodiments, the filter arrangement is designed to attenuate the driver radiation by at least a factor of 10⁶ or 10⁸. Strong suppression of the driver radiation allows for a low-noise output radiation.

[0046] Optionally, the device can also include a polarization filter arrangement which is arranged in a beam path of the output radiation 22, for example as in Fig. 1 The polarization filter arrangement is shown for filter arrangement 78. The polarization filter arrangement can be implemented alternatively or additionally to filter arrangement 78, or combined with it. The polarization filter arrangement suppresses electromagnetic radiation with a polarization perpendicular to a polarization direction of the output radiation.

[0047] For example, the output radiation can be linearly polarized. The polarization filter arrangement can therefore be configured to transmit electromagnetic radiation with a polarization direction parallel to the polarization direction of the output radiation and to suppress electromagnetic radiation with a polarization direction perpendicular to the polarization direction of the output radiation. For example, the polarization filter arrangement can be implemented in embodiments where the nonlinear medium has or consists of KTA.

[0048] It has been found that in nonlinear media, particularly in KTA, conversion or emission processes can occur, generating noise photons polarized perpendicular to the output radiation. These photons can be suppressed by a polarization filter in the output radiation path, thus improving the signal-to-noise ratio of the output radiation.

[0049] Furthermore, the device 10 can optionally include a radiation source 70, e.g., a laser, which provides electromagnetic radiation to supply the driver radiation 23. For this purpose, the electromagnetic radiation from the radiation source 70 can be coupled into a resonator, as described in relation to Fig. 6 As described. In alternative embodiments, the radiation source 70 can provide the driver radiation 23, i.e., at full intensity, without the propagation path 30 needing to be located in a resonator or cavity within the nonlinear medium 40. In both cases, the driver radiation 23 can be provided with high power, e.g., more than 100 W, high beam quality, e.g., M 2 < < 1.5, and a narrow spectral bandwidth, e.g., < 100 MHz. The use of a resonator allows for a lower output power of the radiation source, thus saving costs. In other words, increasing the size of the resonator reduces the required laser power and therefore the system price. Conversely, using a radiation source that provides the driver radiation 23 directly, without an external resonator, allows for a simpler, resonator-free device.

[0050] Free propagation can be understood, for example, as propagation without waveguiding. This can mean, for instance, that the nonlinear medium has no waveguide structure and / or that the nonlinear medium is not part of such a structure. Examples of propagation of the driver radiation 23 in the nonlinear medium are given below with regard to Fig. 2 described, whereby the properties of the beam paths described therein may optionally also apply to the input radiation 21 and the output radiation 22.

[0051] Fig. 2 Figure 1 illustrates a beam path of the driver radiation 23, the input radiation 21, and the output radiation 22 according to an exemplary embodiment. As shown in Figure 2 Fig. 2 As shown, the driver radiation 23 and the input radiation 21 can enter the nonlinear medium 40 through an inlet facet 42. The driver radiation 23 and the input radiation 21 can be focused on a focal point 45, which designates a position along the propagation path 30. The driver radiation 23 and the output radiation 22 can then exit the nonlinear medium 40 through an outlet facet 44. The inlet facet 42 can have an aperture 43. This aperture can be, for example, circular, oval, or rectangular. In some examples, the nonlinear medium 40 is cylindrical or cuboidal, so that the aperture of the outlet facet 44 is the same as the aperture of the inlet facet 42. However, in other examples, the aperture of the outlet facet 44 can have a different size than the aperture of the inlet facet 42.The following description assumes a circular beam cross-section for both the driver radiation 23 and the input radiation 21, so that the discussed quantity relationships apply to all spatial directions perpendicular to the propagation path 30. However, the beam cross-section is not necessarily circular, but can, for example, also be oval or have another shape. The discussed quantity relationships then apply to the consideration of a specific spatial direction perpendicular to the propagation path 30.

[0052] At focal point 45, the driver radiation has a cross-sectional area 47, also denoted as 2*w 0, where w 0 can be the radius in the case of a circular cross-sectional area. The cross-sectional area 47 of the driver radiation 23 can be different from, or identical to, the cross-sectional area of ​​the input radiation 21 at focal point 45. The smaller the cross-sectional area 47, the higher the power density of the driver radiation 23 at focal point 45, and the higher the probability of the conversion process. As in Fig. 2 As shown, the driver radiation 23 can diverge after passing through the focal point 45 and leave the nonlinear medium through the exit facet 44.

[0053] According to the exemplary embodiments, free propagation of the driver field and / or the input radiation 21 takes place in the nonlinear medium 40. Therefore, no wave guidance occurs.

[0054] In exemplary embodiments, a length 48 of the nonlinear medium 40 along the direction of the propagation path 30 satisfies the following condition: L ≤ z R ∗ 1 3 D w 0 2 − 4

[0055] Here, w 0 denotes the beam cross-section 47 of the driver radiation 23 and D the aperture 43 of the entrance facet 42 and / or the exit facet 44. As from Fig. 2 As can be seen, such a length 48 of the nonlinear medium 40 ensures that a beam diameter 51 at the inlet facet 42 or the outlet facet 44 is so small that it can pass through the aperture 43 without diffraction. Thus, freely propagating driver radiation 23 in the nonlinear medium is not reflected at an interface of the nonlinear medium 40 that differs from the outlet facet 44, so no waveguiding occurs. If the inlet and outlet facets have different aperture sizes, the aperture D can, for example, refer to the smaller aperture of an inlet aperture and an outlet aperture of the nonlinear medium, or to an average of the apertures of the inlet and outlet facets.

[0056] The degree of focusing of the driver radiation 23 on the focal point 45, or the divergence of the driver radiation 23 originating from the focal point 45, i.e., the opening angle of the driver radiation 23 viewed upstream and downstream from the focal point, can be approximately described, for example, by means of the Rayleigh length 53. According to exemplary embodiments, the Rayleigh length 53 of the driver radiation 23 at the focal point 45 is at most the Rayleigh length that a Gaussian beam with the same wavelength and the same beam cross-section 47 would have at the focal point 45 as the driver radiation 23. This can be true viewed from the focal point 45 along the beam direction and against the beam direction.

[0057] According to exemplary embodiments, the length 48 of the nonlinear medium 40 is at least 2 mm. As shown from Fig. 2 As can be seen, due to the beam's widening from the focal point 45, the beam cross-section 51 increases with increasing distance from the focal point 45. A greater length of the nonlinear medium 40 thus allows for a larger beam cross-section 51 at the inlet facet 42 and the outlet facet 44. With a constant power density at the focal point 45, a greater length 48 can therefore reduce the power density at the inlet facet 42 and the outlet facet 44. The surfaces of the nonlinear medium can be particularly susceptible to damage from high power densities. A length of at least 2 mm for the nonlinear medium 40 ensures that the power density at the inlet facet 42 and the outlet facet 44 remains sufficiently low.

[0058] For example, the diameter D of the aperture 43, or more generally a size D of the aperture 43, is at least: D ≥ 12 P πI D , where P is the power of the driver array and I D The destruction threshold. With P = 100 W and the values ​​for I D (according to

[25] , thermals are dominant in the ns-pulse regime and in the high reprate regime, which is why the destruction thresholds are assumed to be transferable to the cw regime), in particular, the aperture 43 for the following materials of the nonlinear medium 40 can be configured as follows: BBO: D >= 11.5 µm

[26] , LBO: D >= 4 µm

[27] , KTA and KTP: D >= 9 µm

[28] , LN: D >= 18 µm

[29] , MgO:LN: D >= 100 µm

[30] , LiTa: D >= 13 µm

[29] , MgO:LiTa: D >= 260 µm

[31] .

[0059] Furthermore, according to exemplary embodiments, the length 48 of the nonlinear medium 40 is at most 60 mm. Due to the different wavelengths of the driver radiation, the input radiation, and the output radiation, phase matching of the radiations becomes less accurate with increasing distance. A length of less than 60 mm allows for precise phase matching over the length of the nonlinear medium.

[0060] According to exemplary embodiments, the aperture 43, for example at least the smaller of the apertures of the inlet facet and the outlet facet, is between 0.5 mm and 10 mm. As already mentioned, the beam cross-section does not necessarily have to be circular. Accordingly, the specified size range for the aperture applies independently to each spatial direction perpendicular to the propagation path 30. For example, the specified size range can apply independently to both of two principal axes of an oval aperture or to two edge lengths of a rectangular aperture.

[0061] In other words, the nonlinear medium 40 can be designed in lengths between 2 mm and 60 mm in both cylindrical and oval, in particular circular, geometries according to exemplary embodiments, whereby a size range of 0.5 mm to 10 mm may be advantageous for the edge lengths of the aperture or the principal axes of the aperture.

[0062] According to exemplary embodiments, the power density of the driver radiation 23 at the focus 45 is at least 2 / π · 10⁵ < W / cm², or at least 2 / π · 10⁶ < W / cm². The power density can, for example, refer to the time-averaged power per unit area of ​​the beam cross-section perpendicular to the propagation path 30 at the focal point 45. The boundaries of the beam cross-section can, for example, be defined by the line along which the intensity has dropped to 1 / e² < the intensity at the beam center, i.e., on the propagation path 30. The latter can also apply to the preceding considerations.

[0063] According to the exemplary embodiments, the focus radius w 0 and the power of the driver radiation 23, P drive are selected such that at least one of the following conditions is met: P drive > 2 ⋅ 10 5 W / cm 2 ⋅ w 0 drive 2 oder P drive > 2 ⋅ 10 6 W / cm 2 ⋅ w 0 drive 2

[0064] According to exemplary embodiments, the device 10 is designed to provide the driver radiation 23 in continuous wave operation. This ensures that at any given time an incoming photon of the input radiation 21 can be converted into a photon of the output radiation 22.

[0065] Fig. 3 illustrates an embodiment of the nonlinear medium 40. According to the in Fig. 3 In the illustrated embodiment, the inlet facet 42 of the nonlinear medium 40 forms an angle α in a range of 0° to 10° with a plane perpendicular to the propagation path 30. Furthermore, the outlet facet 44 of the nonlinear medium 40 forms an angle α' in a range of 0° to 10° with a plane perpendicular to the propagation path 30. In some examples, the magnitude of angle α can be the same as the magnitude of angle α'. In this case, the inlet facet 42 and the outlet facet can therefore be parallel to each other. In particular, according to some embodiments, the angle α and the angle α' can be greater than 0°. In this case, electromagnetic radiation reflected by the inlet facet 42 or the outlet facet 44 is reflected in such a way that it leaves an optical path along which the driver radiation 23 propagates.

[0066] Fig. 4 Figure 40 illustrates an orientation of the propagation path 30 with respect to crystallographic axes of the nonlinear medium according to exemplary embodiments. For example, the nonlinear medium 40 has or consists of a material with a crystal structure. The crystal structure can be described by means of crystallographic axes. The material of the nonlinear medium 40 can be an optically anisotropic medium. In some examples, the nonlinear medium can have one (i.e., exactly one) optical axis. In other examples, the nonlinear medium can have two optical axes. Examples of materials of the nonlinear medium 40 that have one optical axis are beta-barium borate (BBO), beta-BaB₂O₄, lithium niobate (LN), LiNbO₃ or MgO:LiNbO₃, and lithium tantalate, LiTaO₃ (LT) or MgO:LiTaO₃ (MLT).Examples of materials for the nonlinear medium 40 with two optical axes for the nonlinear medium are potassium titanyl arsenate (KTA), KTiOAsO 4 , potassium titanyl phosphate (KTP), KTiOPO 4 , lithium triborate (LBO), LiB 3 O 5 , and bismuth triborate (BiBO), BiB 3 O 6 .

[0067] According to exemplary embodiments, the nonlinear medium 40 is configured such that the propagation path 30 has a specific orientation with respect to the crystallographic axes of the nonlinear medium material. With these specific orientations, a high efficiency of the conversion process can be achieved, in particular an efficiency on the order of quantum efficiency. Due to the particularly high efficiency when using these specific orientations, a lower laser power can also be used, resulting in a lower risk of damage to the optical components and a lower thermal energy input into the NLM. This, for example, improves the stability of the cavity control, as with regard to Fig. 8 described, reduced.

[0068] According to embodiments in which the material of the nonlinear medium 40 has one, that is, exactly one, optical axis, e.g., BBO, LT, MLT, or LN, the nonlinear medium is configured such that the optical axis forms an angle θ with the propagation path 30. In these cases, the optical axis coincides with a first crystallographic axis 61 of the nonlinear medium 40. The first crystallographic axis is referred to, for example, as the z-axis or c-axis in these materials.

[0069] In embodiments where the material of the nonlinear medium 40 has two optical axes, for example, in examples where the material of the nonlinear medium 40 is KTA, KTP, LBO, or BiBO, the nonlinear medium 40 is configured such that a first crystallographic axis 61 of the nonlinear medium 40 forms an angle θ with the propagation path 40, and a projection 64 of the propagation path 30 into a plane spanned by a second crystallographic axis 62 (e.g., x-axis) and a third crystallographic axis 63 (e.g., y-axis) forms an angle Φ with the second crystallographic axis 62. The angle Φ can, for example, be measured from the second crystallographic axis in the direction of the third crystallographic axis.The assignment of the crystallographic axes of the nonlinear medium 40 to the first, second and third crystallographic axis is such that the refractive index along the first crystallographic axis 61 (z) is greater than the refractive index along the third crystallographic axis 63 (y), and the refractive index along the third crystallographic axis is greater than the refractive index along the second crystallographic axis 62 (x).

[0070] Since the refractive index can be temperature-dependent, the choice of angles θ and Φ can be related to the temperature T of the nonlinear medium 40 during operation. This, in turn, can depend on the power of the driver array 23. In exemplary embodiments, the device 10 can be configured to maintain the temperature T at a specific temperature T during operation. According to these exemplary embodiments, the operating temperature of the nonlinear medium 40 during operation of the device 10 is in a range between 0 °C and 200 °C.

[0071] According to exemplary embodiments, the angle θ, the angle Φ, and the temperature T of the nonlinear medium 40 during operation of the device 10 are selected as a function of the material of the nonlinear medium 40 such that the following system of equations is satisfied. The first system of equations applies to materials with one optical axis, and the second system of equations applies to materials with two optical axes. Here, n denotes the refractive index as a function of the specified parameters. 1 λ 1 = 1 λ 2 + 1 λ 3 1 λ 1 = 1 λ 2 + 1 λ 3 n λ 1 θ ϕ T λ 1 = n λ 2 θ ϕ T λ 2 + n λ 3 θ ϕ T 3 ,

[0072] According to the embodiments, the angle θ, or the two angles θ and Φ, are selected as described in reference

[14] . Furthermore, according to these embodiments, the refractive index n as a function of temperature T is taken from the following references to solve the above systems of equations: for BBO, references

[15] and

[16] ; for LN, references

[17] and

[18] ; for MgO:LiNbO3, reference

[19] ; ​​for KTP, references

[20] ,

[21] and

[25] ; for LBO, reference

[22] ; for BiBO, reference

[23] ; for KTA, for example, for first wavelength values ​​of λ1 = 313 nm or λ1 = 422 nm, reference

[24] .

[0073] According to exemplary embodiments where the nonlinear medium is 40 KTA, the angle θ is 90° and the following applies to the angle Φ: ϕ = ϕ 0 + ∂ ϕ ∂ λ Δλ + ∂ ϕ ∂ λ ∂ λ ∂ T ΔT

[0074] Here, T = 80°C + ΔT; λ = 1064 nm + Δλ; and ϕ₀ = 33° ± 1°. Furthermore, the sum of the reciprocals of the second and third wavelengths equals the reciprocal of the first wavelength, and the second wavelength is smaller than the third wavelength. According to a first alternative, the first wavelength lies in the range of 350 nm to 840 nm, and the third wavelength in the range of 1260 nm to 1675 nm. The following applies to the equation above. ∂ ϕ ∂ λ = − 0 , 9 ° / nm ± 0 , 3 ° / nm ; and ∂ λ ∂ T = 0 , 05 nm / ° C ± 0 , 025 nm / ° C According to a second alternative, the first wavelength lies in the range of 600 nm to 700 nm and the third wavelength in the range of 1260 nm to 1675 nm. According to a third alternative, the first wavelength lies in the range of 635 nm to 640 nm and the third wavelength in the range of 1260 nm to 1675 nm. According to a fourth alternative, the first wavelength lies in the range of 635 nm to 640 nm and the third wavelength in the range of 1530 nm to 1675 nm. The above equation applies to the second through fourth alternatives. ∂ ϕ ∂ λ = − 0 , 9 ° / nm ± 0 , 1 ° / nm and ∂ λ ∂ T = 0 , 05 nm / ° C ± 0 , 015 nm / ° C .

[0075] According to exemplary embodiments, a combination of the first wavelength λ₁, the material of the nonlinear medium 40, the angle θ (or, in the case of a nonlinear medium with two optical axes, the angle Φ), and the second wavelength λ₂ of the driver radiation 23 corresponds to one of the combinations specified in Tables 1 to 3. Table 1 relates to exemplary embodiments in which the first wavelength is 313 nm, Table 2 to exemplary embodiments in which the first wavelength is 422 nm, and Table 3 to exemplary embodiments in which the first wavelength is 637 nm. For each of the specified combinations, the second wavelength is a wavelength from the respective range indicated. For example, the second wavelength can be selected according to a desired wavelength of the output radiation 22. In some examples, the first wavelength can be one from a range of ±2 nm around the specified value.The angles θ and Φ given in Tables 1 to 3 each represent an angle range of + / - 5° around the specified value. That is, angle θ and angle Φ can each be an angle from a range of + / - 5° and the value specified for the combination.

[0076] According to further embodiments, a combination of the first wavelength, the material of the nonlinear medium 40, the angle θ, in the case of a nonlinear medium with two optical axes the angle Φ, the second wavelength λ 2 and additionally the temperature T corresponds to one of the combinations given in Tables 1 to 3.

[0077] For example, the combinations listed in Tables 1 to 3 can be used to convert electromagnetic radiation from quantum emitters emitting at the respective first wavelength λ1 into a spectral range relevant for telecommunications. For example, quantum emitters implemented using Be+ ion traps (see Reference [8]) emit in a spectral range around 313 nm. In this case, conversion to the telecommunications frequency range of 1260 nm to 1675 nm can be achieved using a driver field 23 at a wavelength between 388 nm and 423 nm. In an alternative embodiment, the first wavelength is 422 nm, which can be converted to the aforementioned telecommunications frequency range using a driver field at a wavelength of 570 nm to 630 nm.In other embodiments, the first wavelength is 637 nm, for example, in the implementation of quantum emitters using electron spin qubits in nitrogen vacancy centers in diamond, see reference [9]. Using a driver field 23 at a wavelength between 1050 nm and 1275 nm, this wavelength can be converted into the aforementioned telecommunications frequency range. Table 1: λ 1 = 313 nm NLM BBO BiBO BiBO BiBO BiBO BiBO θ / ° 35 60 70 80 90 140 ϕ / ° - 0 0 0 0 90 T / °C 0 - 200 0 - 200 0 - 200 38 - 200 119 - 200 0 - 200 λ 2 / nm 388 - 423 388 - 423 388 - 423 416 - 423 418 - 423 388 - 423 NLM BiBO LBO LBO LBO LBO LBO θ / ° 150 5 15 25 35 45 ϕ / ° 90 90 90 90 90 90 T / °C 0 - 180 0 - 116 0 - 200 0 - 200 0-6 6 - 200 λ 2 / nm 388 - 403 388 - 399 388 - 404 389 - 413 413 - 423 398 - 423 NLM LBO LBO LBO LBO θ / ° 55 90 5 15 ϕ / ° 90 37 0 0 T / °C 178 - 200 0 - 200 0 - 114 0-47 λ 2 / nm 422 - 423 388 - 423 388 - 397 388 - 391 Table 2: λ 1 = 422 NLM BBO BBO BiBO BiBO BiBO BiBO BiBO BiBO θ / ° 29,9 50 49,5 59,5 157,8 90 90 90 ϕ / ° - - 0 0 90 55 65 75 T / °C 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 131 0 - 189 0 - 200 λ 2 / nm 570 - 630 570 - 630 570 - 630 597 - 630 570 - 630 604 - 630 587 - 630 577 - 630 NLM BiBO KTA KTA KTA KTA KTA KTA KTA θ / ° 90 81,6 90 90 90 90 90 90 ϕ / ° 85 0 0 10 20 30 40 50 T / °C 0 - 200 0 - 142 0 - 156 0 - 156 0 - 200 0 - 200 0 - 200 0 - 200 λ 2 / nm 573 - 622 570 - 582 570 - 583 570 - 585 570 - 589 574 - 589 580 - 600 587 - 606 NLM KTA KTA KTA KTA KTA KTA KTA KTA θ / ° 90 90 90 90 65 75 85 68,4 ϕ / ° 60 70 80 90 90 90 90 90 T / °C 0 - 200 0 - 200 0 - 200 0-195 0 - 200 0 - 200 0 - 200 0 - 200 λ 2 / nm 595 - 612 602 - 616 607 - 618 610 - 619 570 - 601 583 - 611 603 - 619 570 - 598 NLM KTP KTP KTP KTP KTP KTP KTP KTP θ / ° 75 85 60 70 90 90 90 90 ϕ / ° 0 0 90 90 5 15 25 35 T / °C 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 λ 2 / nm 570 - 595 592 - 604 570 - 608 608 - 630 601 - 608 603 - 614 607 - 618 615 - 627 NLM KTP LBO LBO LBO LBO LBO LBO LBO θ / ° 90 5 15 25 5 15 25 90 ϕ / ° 45 90 90 90 0 0 0 19 T / °C 0 - 200 129 - 200 130 - 200 194 - 200 0 - 200 0 - 200 0-75 0 - 200 λ 2 / nm 625 - 630 602 - 630 609 - 630 629 - 630 601 - 630 570 - 630 570 - 587 570 - 630 Table 3: λ 1 = 637 nm NLM BBO BBO BiBO BiBO BiBO KTA KTA KTA θ / ° 24 34 5 40 50,4 90 90 90 ϕ / ° - - 0 0 0 5 15 25 T / °C 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 λ 2 / nm 1050 - 1275 1050 - 1275 1050 - 1275 1050 - 1275 1050 - 1275 1080 - 1097 1072 - 1093 1061 - 1084 NLM KTA KTA KTA KTA KTA KTA KTA KTA θ / ° 90 90 55 65 75 85 50 60 ϕ / ° 35 45 0 0 0 0 90 90 T / °C 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 λ 2 / nm 1050 - 1070 1034 - 1054 1060 - 1160 1143 - 1287 1113 - 1233 1073 - 1115 1038 - 1202 1143 - 1275 NLM KTA KTP KTP KTP KTP KTP KTP LBO θ / ° 70 57 67 77 48 58 68 5 ϕ / ° 90 0 0 0 90 90 90 90 T / °C 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 0 - 200 λ 2 / nm 1038 - 1152 1050 - 1275 1091 - 1275 1050 - 1100 1050 - 1275 1056 - 1244 1050 - 1056 1137 - 1275 NLM LBO LBO LBO LBO LBO LBO LBO LBO θ / ° 15 25 35 45 55 5 15 25 ϕ / ° 90 90 90 90 90 0 0 0 T / °C 0 - 144 0 - 200 0 - 200 0 - 200 79 - 200 0 - 200 0 - 200 0-45 λ 2 / nm 1189 - 1275 1134 - 1275 1133 - 1237 1050 - 1151 1050 - 1085 1080 - 1275 1050 - 1244 1050 - 1079 NLM LBO LBO LBO LN θ / ° 75 85 90 58 ϕ / ° 0 0 5 - T / °C 152 - 200 1 - 167 0 - 18 0 - 200 λ 2 / nm 1050 - 1275 1050 - 1275 1050 - 1275 1050 - 1275

[0078] Alternatively, the conversion process can be implemented outside the specified angular ranges, which also achieves a low noise rate. In this case, a higher power output from the driver array can be used to achieve a comparable conversion efficiency. This can be advantageously achieved, for example, using pump recycling; see, for example, [reference]. Fig. 8 , will be implemented.

[0079] Fig. 5 Figure 1 illustrates another embodiment of the nonlinear medium 40, according to which the second-order electrical susceptibility of the nonlinear medium 40 exhibits a periodically alternating sign along the propagation path 30. According to these embodiments, the NLM 40 thus exhibits a periodically alternating polarity. For example, in Fig. 5 The first areas 71 exhibit a first polarity and the second areas 73 exhibit a second polarity.

[0080] According to exemplary embodiments with a periodically polarized nonlinear medium, the [context] with respect to Fig. 4 The described angle θ should be 90°. The angle Φ can be chosen arbitrarily in these examples. In other words, the propagation can occur in the direction where the effective nonlinearity deff is maximal.

[0081] According to exemplary embodiments, a combination of the material of the nonlinear medium 40 and a period γ of periodically alternating polarity is one of those listed in Tables 4, 5, or 6. Table 4 refers, for example, to a value for the first wavelength λ 1 of 422 nm, Table 5 to a value for the first wavelength λ 1 of 637 nm, and Table 6 to a value for the first wavelength λ 1 of 313 nm. Table 4 NLM KTA KTP LN LT und MLT γ / µm 4 bis 5 4,8 bis 5,5 3 bis 4,5 3,9 bis 4,8 Table 5 KTA KTP LN LT und MLT γ / µm 14,5 bis 15,5 15 bis 16,5 11,5 bis 12,5 12,7 bis 13,9 Table 6 NLM LT / MLT γ / µm 1,4 bis 1,9

[0082] In other words, the frequency conversion of photons into the infrared spectral range relevant for telecommunications, from 1260 nm to 1675 nm, can be achieved according to embodiments of the present invention by a second-order nonlinear conversion process (three-wave mixing) in a medium that does not contain a waveguide structure. Both periodically polarized and unpolarized (or uniformly polarized) bulk crystals, i.e., crystals without a waveguide structure, can be used.

[0083] Fig. 6 Figure 10 illustrates an example of the device, according to which the device 10 further comprises a resonator 60 which is resonant for the second wavelength λ₂. For example, the length of an optical path of the resonator 60 corresponds to an integer multiple of the second wavelength. The propagation path 30 in the nonlinear medium 40 is arranged in the resonator 60, for example in an optical path of the resonator 60, to provide the driver radiation 23 along the propagation path 30.

[0084] The propagation path 30 can therefore be part of an optical path of the resonator 60. For example, the resonator 60 is configured as a ring resonator. In the resonator 60, a resonator input radiation 19 coupled into the resonator can be superimposed on itself multiple times, so that the power of the driver radiation 23 in the resonator 60 can be several times the power of the resonator input radiation 19. In other words, the driver radiation, also called the driver field, can be generated by the resonant amplification of a lower-power laser beam 19 in a cavity, i.e., the resonator. The representation of the resonator 60 in Fig. 6 This is to be understood schematically. For example, the resonator input radiation 19 can be provided by the radiation source 70.

[0085] Fig. 7 illustrates examples of the resonator 60 as they can be implemented in embodiments of the device 10. Fig. 7a Illustrates an example of the resonator 60 as a ring resonator with four mirrors M1-M4. Fig. 7b Illustrates an example of the resonator 60 as a ring resonator with three mirrors M1-M3. Fig. 7c Illustrates an example of the resonator 60 as a ring resonator with two mirrors M1 and M2 and prism P.

[0086] In further embodiments, the resonator 60 can be implemented using a ring resonator within the nonlinear medium 40. In further embodiments, the resonator 60 is implemented as a resonator such as those used in so-called "whispering-gallery lasers", see reference

[10] , or nonpolar ring oscillators, see reference

[11] .

[0087] According to exemplary embodiments, the resonance condition of the resonator 60 is fulfilled for the driver radiation 23, but not for the input radiation 21 and the output radiation 22. This means, for example, that the resonator 60 is resonant for the wavelength of the driver radiation 23 and not resonant for the wavelengths λ1 and λ3 of the input radiation 21 and the output radiation 22. In these examples, the resonance condition of the resonator 60 is fulfilled exclusively for the driver field 23.

[0088] According to exemplary embodiments, the mirrors of the in Fig. 7 The resonators 60 shown are coated with mono- or dichroic highly reflective layer systems. All of the optics functioning as resonator mirrors, i.e., mirrors M1-M4, M1-M3, M1, M2, and P, can be designed to be highly reflective (HR) for the driver field 23. Highly reflective can refer to a reflection coefficient of RHR > 99%.

[0089] The input radiation 21 can be coupled into the optical resonator 60 at one of the mirrors M#. Depending on which mirror this is implemented at, the input radiation is reflected at none, one, or several of the optical elements M#, P acting as mirrors before entering the nonlinear medium 40. Each of the optics at which the input radiation 21 is reflected before entering the nonlinear medium 40 can, according to exemplary embodiments, be provided with a high-transmittance (HR) coating for the first wavelength λ1. The mirror at which the input radiation 21 is coupled can also be provided with a high-transmittance (HT) coating, e.g., on both sides. In the high-transmittance case, the reflection coefficient RHT can be < 1% for the first wavelength. Similarly, according to exemplary embodiments, the output radiation 22 can be coupled out of the optical path of the resonator 60 at one of the mirrors.This mirror can be provided with an HT coating for the third wavelength λ3 of the output radiation 22, e.g., on both sides. The output radiation 22 can be reflected by none, one, or more of the optical elements acting as mirrors before exiting the optical path of the resonator 60. These elements can be coated with an HR coating for the third wavelength λ3.

[0090] Fig. 8 illustrates examples of the radiation paths of the driver radiation 23, the input radiation 21 and the output radiation 22 for the in Fig. 7a The example shown is of the Resonator 60 with four mirrors. The in Fig. 8 The beam paths shown represent the exemplary implementations with reflection at the maximum possible number of mirrors for this implementation of the resonator 60. Fig. 8a Figure 1 illustrates an example of a beam path for the driver radiation 23. According to this embodiment, the four optics M1-M4 are coated with an HR coating on the side facing the resonator, i.e., the side where the optical path of the resonator 60 is reflected. Furthermore, one of the optics, namely optic M2, at which the resonator input radiation 19 is coupled into the resonator 60, is coated with HR on the side facing away from the resonator. Additionally, the entrance facet 42 and the exit facet 44 of the nonlinear medium 40 can be coated with HT and AR (antireflection) coatings, respectively. Fig. 8b Figure 1 illustrates an example of a beam path for the output radiation 22. According to this example, the output radiation 22 is coupled out of the resonator 60 at optic M1 after being reflected at mirrors M2-M4. In this example, optics M2-M3 have HR coatings for the third wavelength λ3 on the side facing the resonator 60. Furthermore, optic M1 has an HT coating on both the side facing the resonator and the side facing away from the resonator. Alternatively, the output radiation 22 can also be coupled out at optic M2, optic M3, or optic M4. In these cases, the HR coating for the third wavelength λ3 can be omitted on the optics where the output radiation 22 is not reflected. Furthermore, the exit facet 44 of the nonlinear medium 40 can be coated on its rear side for the third wavelength HT in the beam direction. Fig. 8c Figure 1 illustrates an example of a beam path for the input radiation 21. According to this example, the input radiation 21 is coupled to optic M2, which has an HT coating for the first wavelength on both the side facing the resonator and the side facing away from the resonator. In this case, the input radiation 21 is reflected by optics M3, M4, and M1 before entering the nonlinear medium 40. The optics from which the input radiation 21 is reflected have an HR coating for the first wavelength λ1. Furthermore, the entrance facet 42 can have an HT coating for the first wavelength λ1. In alternative examples, the input radiation 21 can be coupled to M3 followed by reflection at M4 and M1, or it can be coupled to M4 followed by reflection at M1, or it can be coupled to M1. In these cases, the step shown in Figure 2 is omitted. Fig. 8c The HR coating shown is applied to the optics where no reflection occurs. The optic where the input radiation is coupled in can have HT coatings on both sides.

[0091] Regardless of the specific implementation of the resonator 60, the device can be configured to control an optical path length of the resonator 60 to a multiple of the second wavelength.

[0092] In other words, the device 10 can be configured to actively maintain the resonance condition of the resonator 60 for the second wavelength. In other words, to maintain the cavity's resonance condition over time, the optical path length can be controlled via a feedback loop to an integer multiple of the wavelength of the radiation to be amplified. This can be implemented, for example, by mounting a cavity mirror on a piezoelectric element. This could, for instance, be one of the mirrors in Fig. 7 or Fig. 8 The piezoelectric element should preferably not be HT-coated on the side facing away from the resonator. Alternatively, an optical element can be inserted into the beam path whose refractive index can be modulated by an electrical signal (e.g., acousto-optic modulator, Pockels cell, etc.). To generate this electrical signal, or a signal for controlling the piezoelectric element (control signal), a PID controller can be used, for example, which converts a so-called error signal into a control signal. The error signal can be generated in various ways (e.g., Pound-Drever-Hall method

[12] , Hänsch-Couillaud method

[13] , etc.).

[0093] Fig. 9 Figure 1 illustrates another example of the device 10. According to this embodiment, the device 10 further comprises a further resonator 60'. The further resonator 60' is also resonant for the second wavelength λ₂ of the driver radiation 23. A further propagation path of a further nonlinear medium 40' is arranged in the resonator 60'. According to this embodiment, the device 10 is configured to superimpose a further electromagnetic input radiation 21' with a further electromagnetic driver radiation 23' of wavelength λ₂ along the propagation path of the further nonlinear medium 40' in order to generate a further electromagnetic output radiation 22'. According to these embodiments, the device 10 is configured to use a portion 27 of the driver radiation 23 coupled out of the resonator 60 to provide the further driver radiation 23' in the further resonator 60'.

[0094] The portion 27 of the driver radiation 23 coupled out of resonator 60 can therefore be used to provide a resonator input radiation 19' of the further resonator 60'. For example, the further input radiation 21' can have the first wavelength λ1 of the input radiation 23, and the further output radiation 22' can have the wavelength λ3 of the output radiation 22. Thus, resonator 60 and the NLM 40 can, for example, be part of a first frequency converter module, and the further resonator 60' and the NLM 40' can be part of a second frequency converter module. The device 10 can be extended by further frequency converter modules, wherein the portion of the driver radiation coupled out of one of the frequency converter modules can be used for one or more of the other frequency converter modules.

[0095] In other words, the driver radiation 27 transmitted through the enhancement cavity 60 can be further utilized according to exemplary embodiments. This can be implemented, for example, by introducing an optical system for collimation and focusing onto an optical fiber in the beam path of the transmitted radiation 23'. The transmitted radiation 23' can then be supplied to a further converter via the optical fiber, for example, the converter of the further nonlinear medium 40' according to [reference to relevant example]. Fig. 8 formed converter. In exemplary embodiments, the device has 10 further converters, i.e., further nonlinear media 40', in which a respective driver radiation is provided by means of further resonators 60'. In other words, the in Fig. 8 The arrangement shown can be cascaded.

[0096] Pump recycling as in Fig. 8 This can be particularly advantageous with regard to the operating and manufacturing costs of the device 10, especially for a plurality of frequency converter modules.

[0097] According to exemplary embodiments, the device 10 further comprises an amplifier 72. The amplifier 72 is designed to amplify the portion 27 of the driver radiation 23 coupled out of the resonator 60. Furthermore, the device 10 can supply the amplified driver radiation to the further resonator 60'. For example, the portion 27 of the driver radiation 23 can be amplified such that the driver radiation 23' in the further resonator 60' has the same power as the driver radiation 23 in the resonator 60.

[0098] In other words, the device 10 can be supplemented by a post-amplifier 72 to achieve a constant input power 19 for various converters of a spatially extended transmission chain with multiple nodes, for example quantum processors, registers, repeaters.

[0099] For example, efficient coupling can be achieved by controlling the length of the resonator to an integer multiple of the second wavelength. This ensures that beam components transmitted through the HR-coated mirror interfere constructively, while reflected beam components interfere destructively. With perfect control (and single-frequency radiation), 100% of the incident radiation passes through the HR mirror into the resonator. This is independent of the reflectivity. The resulting peak in the resonator 60 then results from how often the radiation is superimposed on itself within the resonator. The higher the reflectivities of the mirrors (or the lower the losses the radiation experiences during one resonator revolution), the more frequently this occurs. The relationship is (without further losses): I / I_0 = 1 / (1-R_1*R_2*R_3*R_4), where I / I_0 is the superelevation and R_i is the reflectivity of the 4 resonator mirrors.For example, a ring resonator with HR-coated mirrors with R=99.5% results in an increase of 50.

[0100] For example, the transmitted intensity 23' then results from the transmittance of the respective mirrors. In the preceding example, a proportion of 0.5% is transmitted at each mirror. Multiplying this by the power circulating in the resonator, it follows that 0.5% * 50 * I_0 = 0.25 * I_0 is transmitted at each mirror. With perfect control and no losses in the resonator, 100% of the intensity of the radiation 19 is thus transmitted into and out of the resonator. In embodiments of the present invention, pump recycling is particularly efficient because the conversion process does not result in any losses of the resonant radiation field, as a shorter-wavelength photon at the input wavelength is converted into a photon of the driver radiation and a photon of the output wavelength.

[0101] The resonator input radiation 19 of the device 10 according to Fig. 6 , Fig. 8 oder Fig. 8 The resonator input radiation 19 can therefore be provided by a radiation source 70, for example, a laser. Alternatively, the resonator input radiation 19 can be provided by an amplifier. The resonator input radiation 19 is preferably narrowband and can be linearly polarized.

[0102] It should be noted that this is in view of Fig. 9 The described reuse of the driver radiation 23, the "pump recycling", is not limited to embodiments with a resonator, but can also be implemented in embodiments in which the driver radiation 23 is used as described above. Fig. 1 described without the propagation path 30 being arranged in a resonator. For this purpose, the driver radiation 23 can, for example, be provided as driver radiation 23' after passing through the nonlinear medium 40, or optionally amplified by means of the amplifier 72 before being provided as driver radiation 23'.

[0103] Fig. 10 Figure 1 shows a flowchart of a method 100 according to an embodiment. The method 100 involves superimposing an electromagnetic input radiation 21 with a first wavelength λ1 with an electromagnetic driver radiation 23 with a second wavelength λ2 along a propagation path 30 in a nonlinear medium 40 to generate an electromagnetic output radiation with a third wavelength λ3, such that the second wavelength λ2 is longer than the first wavelength, and such that a power density of the driver radiation 23 in a focus 45 of the driver radiation 23 in the nonlinear medium 40 is at least 2 / π · 10⁵ < W / cm² < or at least 2 / π · 10 6< W / cm 2< is.

[0104] Exemplary embodiments of the invention can be used in laboratory setups and industrially for the data transmission of single-photon signals. This applies in particular to fiber-optic networks for entanglement distribution or quantum key distribution. Besides its application at a network node, where the invention converts the fluorescence signal into qubits, it can also be used in so-called trusted repeaters. These are repeater stations that can be entangled with the nodes of the transmitter and receiver to achieve secure data transmission between them.

[0105] Although some aspects of the present invention have been described as features relating to a device, it is clear that such a description can also be considered a description of corresponding process features. Although some aspects have been described as features relating to a process, it is clear that such a description can also be considered a description of corresponding features of a device or the functionality of a device.

[0106] In the preceding detailed description, various features in exemplary embodiments were sometimes grouped together to streamline the invention. This should not be interpreted as an intention that the claimed embodiments have more features than are expressly stated in each claim. Rather, as the following claims reflect, the subject matter may consist of fewer than all the features of a single disclosed example. Consequently, the following claims are hereby incorporated into the detailed description, each claim being able to stand as its own separate example.While each claim can stand as a separate example, it should be noted that, although dependent claims refer back to a specific combination with one or more other claims, other embodiments also include a combination of dependent claims with the subject matter of each other dependent claim, or a combination of each feature with other dependent or independent claims. Such combinations are included unless it is stated that a specific combination is not intended. Furthermore, it is intended that a combination of features of a claim with any other independent claim is also included, even if that claim is not directly dependent on the independent claim.

[0107] The embodiments described above merely illustrate the principles of the present invention. It is understood that modifications and variations of the arrangements and details described herein will be obvious to other people skilled in the art. Therefore, it is intended that the invention be limited only by the scope of protection set forth in the following claims and not by the specific details presented herein by way of description and explanation of the embodiments. Referenzen

[0108] [1] A. Dahlberg, M. Skrzypczyk, T. Coopmans, L. Wubben, F. Rozp dek, M. Pompili, A. Stolk, P. Pawe czak, R. Knegjens, J. d. O. Filho, R. Hanson, S. Wehner 2019,159. [2] Y. Yu, F. Ma, X.-Y. Luo, B. Jing, P.-F. Sun, R.-Z. Fang, C.-W. Yang, H. Liu, M.-Y. Zheng, X.-P. Xie, W.-J. Zhang, L.-X. You, Z. Wang, T.-Y. Chen, Q. Zhang, X.-H. Bao, J.-W. Pan, Nature 2020, 578, 240. [3] R. Ikuta, T. Kobayashi, S. Yasui, S. Miki, T. Yamashita, H. Terai, M. Fujiwara, T. Yamamoto, M. Koashi, M. Sasaki, Z. Wang, N. Imoto, Opt. Express 2014, 22, 11205. [4] A. Dréau, A. Tcheborateva, A. E. Mahdaoui, C. Bonato, R. Hanson, Phys. Rev. Applied 2018, 9. [5] P. C. Strassmann, A. Martin, N. Gisin, M. Afzelius, Opt. Express 2019, 27, 14298. [6] H. Rütz, K.-H. Luo, H. Suche, C. Silberhorn, Appl. Phys. B 2016, 122. [7] Arlee Smith, SNLO_help, https: / / www.as-photonics.com / snlo_files / SNLO_help.pdf 2015 .[8] SL Todaro, VB Verma, KC McCormick, DTC Allcock, RP Mirin, DJ Wineland, SW Nam, AC Wilson, D. Leibfried, DH Slichter, Physical review letters 2021, 126, 10501. [9] L. Robledo, L. Childress, H. Bernien, B. Hensen, PFA Alkemade, R. Hanson, Nature 2011, 477, 574.

[10] SL McCall, AFJ Levi, RE Slusher, SJ Pearton, RA Logan.

[11] TJ Kane, RL Byer, Optics letters 1985, 10, 65 .

[12] R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, H. Ward, Appl. Phys. B 1983, 31, 97.

[13] T. W. Hansch, B. Couillaud, Optics Communications 1980, 35, 441.

[14] Dmitriev, Handbook of Nonlinear Optical Crystals (1999).

[15] Kato, IEEE JQE v22 p1013 (1986); red modification by Zhang, Opt Comm, v184 p485 (2000).

[16] Eimerl, J. Appl. Phys. v62 p1968 (1987).

[17] Edwards, Opt. Quant. Electron. v16 p373 (1984).

[18] Jundt, Opt. Lett. v22, p1553 (1997).

[19] Zelmon, JOSA B v14 p3319 (1997).

[20] Vanherzeele, Appl. Opt. v27 p3314(1988).

[21] Wiechmann, Opt. Lett. v18 p1208 (1993).

[22] Kato, IEEE JQE v30 p2950 (1994).

[23] Umemura et al. Opt. Mater. v30 p532 (2007).

[24] Kato, CLEO 2004.

[25] Kato, Appl. Opt. v41 p5040 (2002).

[26] Koechner, W. (1976). Solid state laser engineering. New York: Springer-Verlag

[27] G.C. Bhar, et. al.A comparative study of laser-induced surface damage thresholds in BBO crystals and effect of impurities, Optical Materials, Volume 27, Issue 1, 2004, Pages 119-123, ISSN 0925-3467.

[28] Kan, H., Zhang, S., Zhai, K., Ma, X., Luo, Y., Hu, M., & Wang, Q. (2016). Femtosecond-induced micostructures in Magnesium- doped Lithium Niobate crystals with high repetition rate. Optics Communications, 361, 175-180.

[29] Zverev, G., Levchuk, E., Pashkov, V., Poryadin, Y. (1972). LASER-RADIATION-INDUCED DAMAGE TO THE SURFACE OF LITHIUM NIOBATE AND TANTALATE SINGLE CRYSTALS, Soviet Journal of Quantum Electronics, 2, 167

[30] Pengchao Hu, Lianhan Zhang, Jing Xiong, Jigang Yin, Chengchun Zhao, Xiaoming He, Yin Hang, Optical properties of MgO doped near-stoichiometric LiTaO3 single crystals, Optical Materials, Volume 33, Issue 11, 2011, Pages 1677-1680, ISSN 0925-3467, https: / / doi.org / 10.1016 / j.optmat.2011.05.007.

[31] Fang, S., et. al. Growth and optical properties of Mg, Fe Co-doped LiTaO3 crystal.Optik, Volume 117, 2006, Pages 72 - 76.

Claims

1. A device (10) for frequency conversion of electromagnetic radiation, the device having a non-linear medium (40), the device (10) being configured to superimpose electromagnetic input radiation (21) of a first wavelength (λ1) in the non-linear medium (40) with electromagnetic driving radiation (23) of a second wavelength (λ2) along a propagation path (30) in order to generate electromagnetic output radiation (22) of a third wavelength (λ3), wherein the second wavelength is greater than the first wavelength, and wherein the non-linear medium (40) is configured for free propagation of the driving radiation (23) in the non-linear medium (40), characterized in that the second wavelength is smaller than the third wavelength.

2. The device (10) according to claim 1, wherein a power of the input radiation (21) is lower than a power of the driving radiation (23), in particular wherein a power of the input radiation (21) is lower than a power of the driving radiation (23) by at least a factor 108.

3. The device (10) according to claim 1 or 2, wherein the device (10) comprises a resonator (60) resonant for the second wavelength, wherein the propagation path (30) of the non-linear medium (40) is arranged in the resonator (60) to provide the driving radiation (23) along the propagation path (30).

4. The device (10) according to any one of the preceding claims, wherein the device (10) is configured to focus the driving radiation (23) in the non-linear medium (40) to a focus (45) having a cross-sectional size (47) of 2*w0, and wherein a length L of the non-linear medium (40) along the propagation path (30) fulfills the following condition: L ≤ z R ∗ 1 3 D w 0 2 − 4 wherein zR denotes the Rayleigh length for the driving radiation (23), and wherein D denotes a diameter of an aperture of the non-linear medium (40).

5. The device (10) according to any one of the preceding claims, wherein the non-linear medium (40) has a length along the propagation path (30) in a range from 2 mm to 60 mm.

6. The device (10) according to any one of the preceding claims, wherein the non-linear medium (40) has a uniform second-order electrical susceptibility along the propagation path (30).

7. The device (10) according to any one of claims 1 to 5, wherein the second-order electrical susceptibility of the non-linear medium (40) has a sign periodically changing along the propagation path (30).

8. The device (10) according to any one of the preceding claims, wherein an inlet facet (42) and an outlet facet (44) of the non-linear medium (40), between which the propagation path (30) extends, each enclose an angle (α) in a range from 2° to 10° with a plane perpendicular to the propagation path (30).

9. The device (10) according to any one of the preceding claims, wherein the third wavelength is in a range from 1260 nm to 1625 nm, wherein the sum of the reciprocals of the second and third wavelengths corresponds to the reciprocal of the first wavelength.

10. The device (10) according to any one of the preceding claims, further comprising: a resonator (60) resonant for the second wavelength, wherein the propagation path (30) of the non-linear medium (40) is arranged in the resonator (60) to provide the driving radiation (23) along the propagation path (30), and a further resonator (60') resonant for the second wavelength, in which a further propagation path (30') is arranged which extends through a further non-linear medium (40'), the device (10) being configured to superimpose further electromagnetic input radiation (21) with further electromagnetic driving radiation (23') of the second wavelength λ2 along the further propagation path (30') in order to generate further electromagnetic output radiation, wherein the device (10) is configured to use a portion of the driving radiation (23) coupled out from the resonator (60) to provide the further driving radiation (23') in the further resonator (60').

11. The device (10) according to any one of the preceding claims, wherein the material of the non-linear medium (40) is one of beta barium borate, beta BaB2O4, lithium niobate, LiNbO3 or MgO:LiNbO3, potassium titanyl arsenate, KTiOAsO4, potassium titanyl phosphate, KTiOPO4, lithium triborate, LiB3O5, and bismuth triborate, BiB3O6.

12. The device (10) according to any one of the preceding claims, wherein the device (10) is configured to maintain a temperature of the non-linear medium (40) at a temperature T during operation, wherein T is between 0 °C and 200 °C, wherein the material of the non-linear medium (40) is potassium titanyl arsenate, KTiOAsO4, and wherein a first crystallographic axis of the non-linear medium (40) encloses an angle θ = 90° with the propagation path (30) and wherein a projection of the propagation path (30) into a plane spanned by a second crystallographic axis and a third crystallographic axis of the non-linear medium (40) encloses an angle Φ with the second crystallographic axis, wherein the refractive index along the first crystallographic axis is greater than the refractive index along the third crystallographic axis, and wherein the refractive index along the third crystallographic axis is greater than the refractive index along the second crystallographic axis, wherein the first wavelength is in a range from 350 nm to 840 nm, wherein the third wavelength is in a range from 1260 nm to 1675 nm, wherein the sum of the reciprocals of the second and third wavelengths corresponds to the reciprocal of the first wavelength, and wherein the second wavelength is smaller than the third wavelength, and wherein the following applies: ϕ = ϕ 0 + ∂ ϕ ∂ λ Δλ + ∂ ϕ ∂ λ ∂ λ ∂ T ΔT with T = 80°C + ΔT; λ = 1064nm + Δλ; ϕ0 = 33° ± 1°; ∂ ϕ ∂ λ = − 0.9 ° / nm ± 0.3 ° / nm; and ∂ λ ∂ T = 0.05 nm / ° C ± 0.025 nm / ° C.

13. The device (10) according to any one of the preceding claims, further comprising a filter arrangement (78) arranged in a beam path of the output radiation, wherein the filter arrangement is configured to transmit the output radiation and to suppress the driving radiation (23), in particular wherein the filter arrangement (78) is configured to attenuate the driving radiation (23) by at least a factor of 106 or by a factor of 108.

14. The device (10) according to any one of the preceding claims, further comprising a polarization filter arrangement arranged in a beam path of the output radiation, wherein the polarization filter arrangement is configured to suppress electromagnetic radiation having a polarization perpendicular to a polarization direction of the output radiation.

15. A method (100) for frequency conversion of electromagnetic radiation, comprising: superpositioning electromagnetic input radiation (21) of a first wavelength (λ1) with electromagnetic driving radiation (23) of a second wavelength (λ2) along a propagation path (30) in a non-linear medium (40) to generate electromagnetic output radiation of a third wavelength (λ3) such that the second wavelength is longer than the first wavelength, such that the second wavelength is smaller than the third wavelength, and such that the driving radiation (23) propagates freely in the non-linear medium (40).

Images